Search

Free-standing, one-dimensional TiO2 nanotube arrays (TNAs) with a disordered surface structure are synthesized on transparent conducting substrates, and their opto-physicochemical properties and photoelectrocatalytic (PEC) performances are examined in detail. A two-step anodization process is used to transplant TNAs grown on titanium foils onto fluorine-doped SnO2 substrates (denoted as W-TNAs), after which they are electrochemically reduced for 20 and 90 s (denoted as B-TNAs-20 and 90, respectively). The as-transplanted W-TNAs exhibit low PEC activities in terms of their photocurrent, oxygen evolution reaction (OER), and oxidations of inorganic and organic substrates (iodide and urea, respectively) under simulated sunlight (AM 1.5; 100 mW cm−2), primarily because of the sluggish charge transfer through the poor electrically conductive TNA framework. The quick electrochemical reduction of the W-TNAs leads to an 8-fold larger photocurrent, while significantly accelerating the OER (by three times) and iodide and urea oxidation reactions (by 2 and ∼20 times, respectively). These enhanced PEC activities of the B-TNAs are attributed to the creation of Ti3+ and associated oxygen vacancies which strengthen their n-type characteristics and thereby increase their electrical conductivity. The time-resolved photoluminescence spectra further reveal that the lifetime (τ) of the photogenerated charge carriers in the B-TNAs (τ = 0.33 ns) is an order of magnitude shorter than that of the W-TNAs (τ = 3.63 ns). The disordered surface exhibits a lower Faradaic efficiency for multi-electron transfer oxidation reactions and a higher Faradaic efficiency for single-electron transfer oxidation reactions compared to the W-TNAs. The detailed surface characterization and PEC mechanism are discussed.

This study reports the synthesis of cadmium sulfide (CdS) nanoparticles on Li + -inserted TiO 2 nanotube array (Li-TNA) to fabricate Li-TNA/CdS heterojunction electrodes for photoelectrochemical (PEC) hydrogen production under air mass (AM) 1.5 light and solar visible light (λ > 420 nm). For fabrication of the heterojunction, Li + is rapidly inserted into TNA pre-grown on Ti foil, and CdS is then photodeposited onto the Li-TNA electrodes for varying deposition times. Surface analyses reveal that sub-100-nm polycrystalline CdS particles partly cover the Li-TNA (length: ∼800 nm, pore diameter: ∼100 nm), enabling the heterojunction to utilize AM 1.5 light as well as visible light. In aqueous solutions of sulfide and sulfite, the Li-TNA/CdS exhibits an incident photon-to-current efficiency (IPCE) of ∼20% (λ = 420 nm) while generating H 2 at a Faradaic efficiency of ∼100%. This PEC performance is superior to that of TNA/CdS, which is attributed to the Li + -enhanced charge transfer at the TNA/CdS interface. Electrochemical impedance analysis shows that the charge-transfer resistance of the TNA is reduced by ∼60% by Li + insertion. Time-resolved photoluminescence decay profiles further reveal that the charge transfer in Li-TNA is completed within 0.8 ns, which is ∼33% faster than that in TNA. The sample surface is analyzed using scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible spectroscopy, and the PEC behavior of the samples is discussed in detail.